Photoplethysmography, or “PPG”, is an optical technique for detecting blood volume changes in a tissue. In this technique, one or more emitters are used to direct light at a tissue and one or more detectors are used to detect the light that is transmitted through the tissue (“transmissive PPG”) or reflected by the tissue (“reflectance PPG”). The volume of blood, or perfusion, of the tissue affects the amount of light that is transmitted or reflected. Thus, the PPG signal varies with changes in the perfusion of the tissue.
The blood volume in a tissue changes with each heartbeat, and so the PPG signal also varies with each heartbeat. Traditionally, this component of the PPG signal is referred to as the “AC component” component of the signal, and is also often referred to as the “pulsatile component.” Blood volume is also affected by other physiological processes in the body, including respiration, venous blood volume, sympathetic and parasympathetic tone and certain pathologies. The changes in the PPG signal due to these and other physiological processes, along with changes in the PPG signal due to noise caused by non-physiological processes such as ambient light and bodily movement, have traditionally been referred to collectively as the “DC component.”
The present inventors have recently extracted specific parameters from the DC component. Traditional sites for monitoring PPG, such as fingers and toes, generally provide a relatively small PPG signal, and the quality of this signal may be negatively impacted by sympathetic innervation in these tissue sites. Thus, the DC component signal from traditional peripheral sites may not be of sufficient strength and quality to effectively separate out the signals from different physiological processes.
Thus, new devices, systems and methods for PPG monitoring may be desirable.